Module A3
Three Substitution Mechanisms: D, A, and I
Concept Overview
Ligand substitution reactions in transition metal complexes follow three major pathways:
- Dissociative (D): A two-step process. The leaving ligand X fully dissociates first, forming a stable intermediate with reduced coordination number. The entering ligand Y then coordinates in a rapid second step.
- Associative (A): A two-step process. The entering ligand Y coordinates first to form an expanded coordination intermediate. The leaving ligand X then departs in a fast second step.
- Interchange (I): A concerted, single-step reaction with no intermediate.
- Id: Dominated by M–X bond stretching (dissociative character).
- Ia: Dominated by M–Y bond forming (associative character).
Key Equations
Dissociative (D) — Saturation Kinetics
Where:
- — rate constant for dissociation of X
- — rate constant for association of Y
Under high [Y]:
Associative (A) — Second-Order
Worked Examples
Deciphering the Rate-Determining Step
For , the rate remains unchanged when [I⁻] is increased from 0.05 M to 2.0 M. Identify the mechanism.
Common Misconceptions
❌ Misconception
The Interchange (I) mechanism has a transient intermediate.
✅ Correction
The I mechanism is a single-step concerted process with no intermediate. If an intermediate has a physical lifetime (>10⁻¹³ s), the reaction is D or A, not I.
Interactive Visual
Watch all three substitution mechanisms animate side-by-side. Use the Play button to step through or auto-play the entire sequence.
Dissociative (D)
CN = 6
6-coordinate octahedral complex [ML₅X]
Associative (A)
CN = 6
6-coordinate octahedral complex [ML₅X]
Interchange (I)
CN = 6
6-coordinate octahedral complex [ML₅X]
Formative Quiz
Question 1
In an Associative (A) mechanism, what is the effect of changing the nucleophilicity of Y?
Question 2
An octahedral complex undergoes substitution with Rate = k[Complex][Y]. What is the mechanism?